The present disclosure pertains to imaging systems and imaging methods, e.g., microwave and millimeter wave energy based imaging, using waveguide assemblies including one or more gated elements.
In the art of microwave imaging (MWI), an object of interest (OI) is illuminated by microwave energy and the scattered fields are collected outside the OI. The collected scattered fields may then be used to reconstruct qualitative, and possibly quantitative images, or interior maps, of the OI that include its location, geometry, shape, magnetic properties, and dielectric properties. The ability to provide quantitative imaging and to utilize non-ionizing radiations associated with MWI make MWI a good candidate for use in many novel applications such as non-destructive testing in industrial applications, non-invasive imaging of biological tissues, remote sensing, geophysical survey of underground objects, and other security and military applications.
Due to the inherent non-linear and ill-posed behavior of the inverse scattering problem used in MWI, a substantial amount of electromagnetic scattering data may need to be collected in order to ensure a robust inversion and quantitatively-accurate image. The need for more data can be satisfied by several approaches such as, e.g., increasing the number of data acquisition points, using different frequencies, collecting multiple field polarizations, etc.
Microwave imaging has been deployed in many biomedical, security, and industrial applications such as breast cancer diagnostics (see, e.g., N. Nikolova, “Microwave imaging for breast cancer,” Microwave Magazine, IEEE, vol. 12, no. 7, pp. 78-94, December 2011), biological tissue imaging (see, e.g., M. Ostadrahimi, P. Mojabi, A. Zakaria, J. LoVetri, and L. Shafai, “Enhancement of Gauss-Newton inversion method for biological tissue imaging,” Microwave Theory and Techniques, IEEE Transactions on, vol. 61, no. 9, pp. 3424-3434, 2013), non-destructive testing and evaluation (see, e.g., R. Zoughi, M. A. AbouKhousa, M. T. A. Ghasr, S. Kharkivskiy, and D. Pommerenke, “Microwave and millimeter wave imaging system,”, U.S. Pat. No. 7,746,266; and M. Ghasr, M. Abou-Khousa, S. Kharkovsky, R. Zoughi, and D. Pommerenke, “Portable real-time microwave camera at 24 GHz,” Antennas and Propagation, IEEE Transactions on, vol. 60, no. 2, pp. 1114-1125, February 2012), and geophysical surveying (see, e.g., A. Abubakar and P. Van Den Berg, “Non-linear three-dimensional inversion of cross-well electrical measurements,” Geophysical prospecting, vol. 48, no. 1, pp. 109-134, 2000).
The basic operation of a MWI system is based on illuminating an object-of-interest (OI) by a transmitting antenna and collecting the scattered fields at various receiving locations. The collected field data may be calibrated and then processed using non-linear inverse scattering algorithms (see, e.g., Q. Fang, P. Meaney, and K. Paulsen, “Viable three-dimensional medical microwave tomography: theory and numerical experiments,” Antennas and Propagation, IEEE Transactions on, vol. 58, no. 2, pp. 449-458, 2010; J. De Zaeytijd, A. Franchois, C. Eyraud, and J. Geffrin, “Full-wave three-dimensional microwave imaging with a regularized Gauss-Newton method theory and experiment,” Antennas and Propagation, IEEE Transactions on, vol. 55, no. 11, pp. 3279-3292, 2007; A. Zakaria, C. Gilmore, and J. LoVetri, “Finite-element contrast source inversion method for microwave imaging,” Inverse Problems, vol. 26, p. 115010, 2010; and P. Mojabi and J. LoVetri, “Microwave biomedical imaging using the multiplicative regularized Gauss-Newton inversion,” Antennas and Wireless Propagation Letters, IEEE, vol. 8, pp. 645-648, 2009) or radar techniques (see, e.g., M. Klemm, I. Craddock, J. Leendertz, A. Preece, and R. Benjamin, “Radar-based breast cancer detection using a hemispherical antenna array experimental results,” Antennas and Propagation, IEEE Transactions on, vol. 57, no. 6, pp. 1692-1704, 2009).
Depending on the application, the imaging results, or outcome, may be either a quantitative reconstruction of the complex dielectric and magnetic profile of the OI that provides information on its shape and location and/or a qualitative image that produces the shadow of the OI. The microwave imaging providing qualitative imaging method may not incur a heavy computational burden (e.g., such as quantitative MWI) and may be accomplished in real-time. Although qualitative imaging may provide some information about the internal structure and composition of an OI, qualitative imagine may not provide the ability to identify materials, such as, e.g., tissues, etc., in a reconstructed image as well as quantitative MWI may be able to provide (e.g. which may be helpful in biomedical and geo-surveying applications). Further, quantitative images can be processed and interpreted by intelligent computer algorithms due to the known values of the dielectric properties of materials and biological tissues, which may accelerate image interpretation by skilled technicians, radiologists, and trained human resources.
In order to obtain a quantitative interior image of an OI, microwave energy should penetrate sufficiently into the object. To reduce reflections from the boundary of the OI, and thus maximize field penetration, the OI may be immersed into a matching fluid (see, e.g., C. Gilmore, A. Zakaria, J. LoVetri, and S. Pistorius, “A study of matching fluid loss in a biomedical microwave tomography system,” Medical physics, vol. 40, p. 023101, 2013). Furthermore, because wave penetration depth is inversely proportional to the frequency of operation, upper limits on the frequency that can be used may exist, especially when imaging biological targets. Further, microwave imaging systems used for biomedical applications may operate up to X-band such as, e.g., 915 MHz (see, e.g., J. Stang, M. Haynes, P. Carson, and M. Moghaddam, “A preclinical system prototype for focused microwave thermal therapy of the breast,” Biomedical Engineering, IEEE Transactions on, 2012, early access), 1.0-2.3 GHz (see, e.g., S. Semenov, J. Kellam, Y. Sizov, A. Nazarov, T. Williams, B. Nair, A. Pavlovsky, V. Posukh, and M. Quinn, “Microwave tomography of extremities: 1. dedicated 2D system and physiological signatures,” Physics in Medicine and Biology, vol. 56, p. 2005, 2011), 2.45 GHz (see, e.g., A. Franchois, A. Joisel, C. Pichot, and J. Bolomey, “Quantitative microwave imaging with a 2.45-GHz planar microwave camera,” Medical Imaging, IEEE Transactions on, vol. 17, no. 4, pp. 550-561, 1998), 0.9-1.5 GHz (see, e.g., P. Meaney, M. Fanning, T. Raynolds, C. Fox, Q. Fang, C. Kogel, S. Poplack, and K. Paulsen, “Initial clinical experience with microwave breast imaging in women with normal mammography,” Academic Radiology, vol. 14, no. 2, pp. 207-218, 2007), 2-8 GHz (see, e.g., E. C. Fear, M. A. Stuchly, “Microwave Detection of Breast Cancer,” Microwave Theory and Techniques, IEEE Transactions on, vol. 48, pp. 1854-1863, November 2000), and/or 4-9 GHz (see, e.g., M. Klemm, I. J. Craddock, J. A. Leendertz, A. Preece, R. Benjamin, “Radar-Based Breast Cancer Detection Using a Hemispherical Antenna Array—Experimental Results,” Antennas and Propagation, IEEE Transactions on, vol. 57, no. 6, pp. 1692-1704, June 2009).
Due to the low operational frequency and the compact size of MWI systems, a target may be located in the near-field region of the antennas. In this region, complicated field distributions may exist due to the presence of some or all polarizations, arbitrary wave impedances, and both propagating as well as evanescent modes. Further, polarization may be utilized in microwave imaging and may not be generally available in other imaging modalities. The use of different polarizations in MWI may require the use of inversion algorithms capable of inverting vector field problems; specialized measurement techniques sensitive to individual polarizations; and proper calibration techniques. The ability to use arbitrary polarizations of electromagnetic energy may further require full-wave computational models of the imaging chamber (see, e.g., M. Ostadrahimi, P. Mojabi, C. Gilmore, A. Zakaria, S. Noghanian, S. Pistorius, and J. LoVetri, “Analysis of incident field modeling and incident/scattered field calibration techniques in microwave tomography,” Antennas and Wireless Propagation Letters, IEEE, vol. 10, pp. 900-903, 2011). Such full-wave modeling of the imaging system may be computationally expensive. Further, the measurement of different polarizations may require sophisticated experimental systems that can differentiate between measured signal polarizations. Still further, associated calibration techniques for full-wave modeling may need to be tailored for each polarization and for the specific measurement system being used.
Due to these challenges, existing imaging systems may collect data only in a two-dimensional (2-D) plane, while measuring only a single field polarization in the near-field region (see, e.g., Q. Fang, P. Meaney, and K. Paulsen, “Viable three-dimensional medical microwave tomography: theory and numerical experiments,” Antennas and Propagation, IEEE Transactions on, vol. 58, no. 2, pp. 449-458, 2010; S. Semenov, J. Kellam, Y. Sizov, A. Nazarov, T. Williams, B. Nair, A. Pavlovsky, V. Posukh, and M. Quinn, “Microwave tomography of extremities: 1. dedicated 2D system and physiological signatures,” Physics in Medicine and Biology, vol. 56, p. 2005, 2011; and T. Henriksson, N. Joachimowicz, C. Conessa, and J. Bolomey, “Quantitative microwave imaging for breast cancer detection using a planar 2.45 GHz system,” Instrumentation and Measurement, IEEE Transactions on, vol. 59, no. 10, pp. 2691-2699, 2010). A few 3-D MWI systems exists that only collect a single field polarization (see, e.g., T. Rubæk, O. Kim, and P. Meincke, “Computational validation of a 3-D microwave imaging system for breast-cancer screening,” Antennas and Propagation, IEEE Transactions on, vol. 57, no. 7, pp. 2105-2115, 2009) or place the antennas in the far-field region and rotate the antennas to collect two field polarizations (see, e.g., J. Geffrin and P. Sabouroux, “Continuing with the Fresnel database: experimental setup and improvements in 3D scattering measurements,” Inverse Problems, vol. 25, p. 024001, 2009). Further, 2-D MWI systems using antennas to directly measure fields have been designed and implemented (see, e.g., C. Gilmore, A. Zakaria, P. Mojabi, M. Ostadrahimi, S. Pistorius, and J. LoVetri, “The University of Manitoba microwave imaging repository: a two-dimensional microwave scattering database for testing inversion and calibration algorithms,” Antennas and Propagation Magazine, IEEE, vol. 53, no. 5, pp. 126-133, October 2011), as well as other systems using remote probes based on the Modulated Scattering Technique (MST) (see, e.g., M. Ostadrahimi, P. Mojabi, S. Noghanian, L. Shafai, S. Pistorius, and J. LoVetri, “A novel microwave tomography system based on the scattering probe technique,” Instrumentation and Measurement, IEEE Transactions on, vol. 61, no. 2, pp. 379-390, February 2012).
In at least one MWI approach, some field measurement probes distributed at various locations have been used to infer the electromagnetic field impinging on their location. By changing/modulating the impedance of each probe, its interaction with the electromagnetic field is changed/modulated. The change/modulation of the interaction may then be detected by an antenna, referred to as the collector antenna, at some distances from the probe. The detected modulated signal at the collector antenna was shown to be proportional to the field at the probe's location, which may be referred to as the Modulated Scattering Technique (MST).
MST-based MWI systems may provide several advantages such as, e.g., accurate near-field measurement, robust calibration, inexpensive experimental implementation, collecting various field polarizations (see, e.g., M. Ostadrahimi, A. Zakaria, J. LoVetri, and L. Shafai, “A near-field dual polarized TE-TM microwave imaging system,” Microwave Theory and Techniques, IEEE Transactions on, vol. 61, no. 3, pp. 1376-1384, 2013), and an increased amount of non-redundant data (see, e.g., M. Ostadrahimi, P Mojabi, S. Noghanian, J. LoVetri, and L. Shafai, “A multiprobe-per-collector modulated scatterer technique for microwave tomography,” Antennas and Wireless Propagation Letters, IEEE, vol. 10, pp. 1445-1448, 2011). One MST-based system may utilize probes that are printed dipoles and consist of 5 p-i-n diodes in series. The impedance of the probes may have two cases: the diodes may be forward biased; and the diodes may be reversed biased. In each case, the probe's perturbation of the electromagnetic field is detected by a collector antenna using a Vector Network Analyzer (VNA) (see, e.g., M. Ostadrahimi, P. Mojabi, S. Noghanian, L. Shafai, S. Pistorius, and J. LoVetri, “A novel microwave tomography system based on the scattering probe technique,” Instrumentation and Measurement, IEEE Transactions on, vol. 61, no. 2, pp. 379-390, February 2012; and M. Ostadrahimi, A. Zakaria, J. LoVetri, and L. Shafai, “A near-field dual polarized TE-TM microwave imaging system,” Microwave Theory and Techniques, IEEE Transactions on, vol. 61, no. 3, pp. 1376-1384, 2013) or a custom-designed coherent receiver (see, e.g., M. Ostadrahimi, M. Asefi, J. LoVetri, G. Bridges, and L. Shafai, “An mst-based microwave tomography system using homodyne receiver,” in IEEE International Symposium on Antennas and Propagation and USNC/URSI National Radio Science Meeting. IEEE, 2013, pp. 1-4).